An optical network system including an OADM (optical add/drop multiplexer) has been known as an optical switch node. An ROADM (Reconfigurable Optical Add/Drop Multiplexer), a type of the OADM, is disclosed in a non-patent document of “HAGIMOTO Kazuo, and two others, “Introduction to Optical Network Becoming More Familiar (15)> (which will be referred to as Non-Patent Document 1 hereinafter). The OADM as its basic configuration is described briefly below.
FIG. 146 is a block diagram illustrating an example of a configuration of a conventional OADM. As illustrated in FIG. 146, the OADM includes an optical SW (which may be used as an abbreviation of a switch hereinafter) setting unit 501, an optical SW unit 502, a demultiplexing unit 503, a multiplexing unit 504, and transmit-receive units 505-1 to 505-N.
FIG. 147 is a diagram illustrating an example of a configuration of a ring optical network system in which the OADMs illustrated in FIG. 146 are connected in a ring shape via transmission lines.
As illustrated in FIG. 147, four OADMs 510A to 510D are installed on land and are connected each other via a ring-shaped transmission 520. In a system illustrated in FIG. 147, a wavelength λ1 is allocated to an optical signal transmitted and received between the OADM 510A and the OADM 510C. A wavelength λ3 is allocated to an optical signal transmitted and received between the OADM 510A and the OADM 510D. A wavelength λ2 is allocated to an optical signal transmitted and received between the OADM 510B and the OADM 510C. And, a wavelength λ4 is allocated to an optical signal transmitted and received between the OADM 510B and the OADM 510D. As described above, different wavelength paths having different wavelengths are set for each point to point.
In a metro network which is established as the metropolitan area optical network, as illustrated in “Key Points of Network for Learner in One Week” (which will be referred to as Non-Patent Document 2), wavelength division multiplexing (WDM) is used from a viewpoint of band usage efficiency. As a network topology, a ring-shaped one is used, for example. FIG. 148 is a diagram illustrating a configuration of a conventional metro network. In the metro network illustrated in FIG. 148, a plurality of reconfigurable optical add/drop multiplexers (ROADMs) 530 are installed as nodes into a ring-shaped optical fiber network 531. Control of optical line switch type is provided using a wavelength path, and an appropriate bandwidth is allocated, by statically setting a wavelength path for each point to point in accordance with an estimated maximum value of a point-to-point traffic volume. In the illustrated example, a path having the wavelength λ1 is set between a point A and a point D; and, λ2, between a point B and the point D. FIG. 149A and FIG. 149B are diagrams each for explaining an operating principle of the conventional metro network using the ROADM as described above. For example, different paths having different wavelengths λ1 to λ3 are set from the points A to C, respectively, to the point D as illustrated in FIG. 149A. The point D thus receives temporally non-synchronous data (Data1 to Data4) from the points A to C as illustrated in FIG. 149B.
In the conventional optical network as described above, because a wavelength path is statically set for each point to point in accordance with the estimated maximum traffic. Improvement of bandwidth when a traffic volume is small becomes a problem to be solved. For example, even if an actual point-to-point traffic volume is smaller than an estimated value, which results in a free device resource or bandwidth, it is not possible to use one free bandwidth with a certain point-to-point wavelength path for a communication between another point to point. Conversely, if a given point-to-point traffic volume becomes larger than estimated, it is not possible to transmit or receive part of data at the point to point, using a wavelength path used in other point to point. Further, the number of wavelengths as much as the number of points to points need to be prepared, which causes a problem that the number of points to points is limited depending on types of wavelengths which the ROADM device can output.
An optical ring network (which may also be referred to as a “ring” where appropriate) has been known which can improve traffic accommodation efficiency by using a WDM technique and a TDM (Time Division Multiplexing) technique. Multistage connection of a plurality of such rings makes it possible to efficiently accommodate traffic in a further wide area.
A non-patent document of “Demonstration of the Interconnection of Two Optical Packet Rings with a Hybrid Optoelectronic Packet Router (Alcatel, ECOC2010)” (which will be referred to as Non-Patent Document 3) proposes a time slot (which may also be abbreviated to a “TS” hereinafter) exchange method between WDM/TDM rings, in a multi-ring network in which a plurality of the rings are connected in multiple stages.
In the conventional method disclosed in Non-Patent Document 3, adjustment of a fiber length between the rings allows time slots to be exchanged at a ring intersection point (a node connecting between rings), without collision between a time slot for communication in an upper ring (which may also be referred to as a first time slot) and a time slot for communication between rings from a lower ring to the upper ring (which may also be referred to as a second time slot).